System and method for performing transformer diagnostics

ABSTRACT

A method for performing diagnostics on a target transformer includes applying a voltage output of a voltage generator to a winding or phase of the target transformer; controlling the voltage generator to output an AC voltage at a first frequency and then a second frequency and measuring first and second excitation currents flowing into the target transformer associated with the first frequency and second frequency, respectively. The method further includes determining an actual excitation current of the target transformer as a function of both the first and second excitation currents, and comparing the actual excitation current of the target transformer to excitation currents associated with one or more benchmark transformers having known electrical characteristics. And when the actual excitation current of the target transformer matches an excitation current of one of benchmark transformers, determining the electrical characteristics of the target transformer to match electrical characteristics of the one benchmark transformer.

RELATED APPLICATIONS

This application is a continuation pursuant to 35 U.S.C. § 120 of U.S.patent application Ser. No. 16/261,309, filed Jan. 29, 2019, which is acontinuation pursuant to 35 U.S.C. § 120 of U.S. patent application Ser.No. 15/436,403, filed Feb. 17, 2017, all of which are entirelyincorporated herein by reference.

BACKGROUND Field

This application relates to a diagnostic system. In particular, thisapplication describes a system and method for performing diagnostics ona transformer.

Description of Related Art

High voltage transformers are utilized in the delivery of power and arecommonly utilized to step down voltages present on transmission lines tovoltages more suitable for residential or commercial areas. Transformersgenerally include one or more primary windings and one or more secondarywindings. In the case of a 3-phase electrical system, the primarywindings may comprise three windings, each of which is coupled to adifferent phase of the electrical system.

The voltage delivered to a load is somewhat dependent on the loaditself. Therefore, some high voltage transformers incorporate aload-tap-changer (LTC) and/or a de-energized tap changer (DETC). LTCsand DETCs are a switch mechanism that facilitate changing the turnsratio of the transformer. LTC/DETCs change position to control itsoutput voltage.

High voltage transformers tend to undergo a large amount of stressduring operation. This is especially the case during periods ofpeak-power usage, such as during warm days in the summer. The stress mayresult in degradation in the performance of the transformer. Forexample, insulation between the windings may degrade; shorts may beginto develop between adjacent turns or windings. Other problems may occur.If these problems persist for long enough, the transformer maycatastrophically fail. This failure may, in turn, cause other componentsof the power system to fail.

In an attempt to minimize these sorts of disruptions, transformerstypically undergo routine diagnostic testing after being installed toascertain whether there are any issues that may lead to an eventualcatastrophic failure. One test employed is to energize the windings orphases of the transformer with an AC voltage and to measure themagnitude of the power frequency component of the excitation currentflowing into the transformer.

Unfortunately, in certain modern power transformers, the capacitivecomponent of the excitation current distorts the expected patterns ofthe measured current making conclusions of diagnostic analysis lesscertain.

BRIEF SUMMARY

Methods, systems, and computer-readable media are provided thatfacilitate performing diagnostics on a transformer.

In one aspect, a method for performing diagnostics on a targettransformer includes applying a voltage output of a voltage generator toa winding or phase of the target transformer; controlling the voltagegenerator to output an AC voltage at a first frequency and then a secondfrequency and measuring first and second excitation currents flowinginto the target transformer associated with the first frequency andsecond frequency, respectively. The method further includes determiningan actual excitation current of the target transformer as a function ofboth the first and second excitation currents, and comparing the actualexcitation current of the target transformer to excitation currentsassociated with one or more benchmark transformers having knownelectrical characteristics. And when the actual excitation current ofthe target transformer matches an excitation current of one of benchmarktransformers, determining the electrical characteristics of the targettransformer to match electrical characteristics of the one benchmarktransformer.

In a second aspect, a system for performing diagnostics on a targettransformer is provided. The system includes a voltage generator thatgenerates an AC voltage at a first frequency and a second frequency, aswitch section configured to selectively apply the AC voltage to one ofa plurality of windings or phases of the target transformer, a currentsensor configured to sense a current flowing through a selected windingor phase of the target transformer, a processor in communication withthe voltage generator, the switch section, and the current sensor; andnon-transitory computer readable media in communicating with theprocessor. The non-transitory computer readable media stores instructioncode that when executed by the processor causes the processor to performacts comprising: a) controlling the switch circuitry to apply a voltageoutput of the voltage generator to a winding or phase of the targettransformer; b) controlling the voltage generator to output an ACvoltage at a first frequency; c) measuring, via the current sensor, afirst excitation current flowing into the target transformer associatedwith the first frequency; d) controlling the voltage generator to outputan AC voltage at a second frequency; e) measuring, via the currentsensor, a second excitation current flowing into the target transformerassociated with the second frequency; f) determining an actualexcitation current of the target transformer as a function of both firstand second excitation currents; g) comparing the determined actualexcitation current of the target transformer to excitation currentsassociated with one or more benchmark transformers having knownelectrical characteristics. When the actual excitation current of thetarget transformer matches an excitation current of one of benchmarktransformers, the instruction code causes the processor to determine theelectrical characteristics of the target transformer to match electricalcharacteristics of the one benchmark transformer.

In a third aspect, a non-transitory machine-readable storage medium thatstores a computer program for performing diagnostics on a targettransformer is provided. The program is executable by the machine andcauses the machine to perform acts of: a) controlling switch circuitryto apply a voltage output of a voltage generator to a winding or phaseof the target transformer; b) controlling the voltage generator tooutput an AC voltage at a first frequency; c) measuring, via a currentsensor, a first excitation current flowing into the target transformerassociated with the first frequency; d) controlling the voltagegenerator to output an AC voltage at a second frequency; e) measuring,via the current sensor, a second excitation current flowing into thetarget transformer associated with the second frequency; f) determiningan actual excitation current of the target transformer as a function ofboth first and second excitation currents; g) comparing the determinedactual excitation current of the target transformer to excitationcurrents associated with one or more benchmark transformers having knownelectrical characteristics; and when the actual excitation current ofthe target transformer matches an excitation current of one of benchmarktransformers, determining the electrical characteristics of the targettransformer to match electrical characteristics of the one benchmarktransformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system for performing diagnostics on atarget transformer;

FIGS. 2A and 2B illustrate exemplary transformer configurations to whichoutputs of the system of FIG. 1 may be coupled;

FIG. 3 illustrates various exemplary operations that may be performed bythe system;

FIG. 4A illustrates an exemplary circuit representation of a phase of atransformer;

FIG. 4B illustrates a vector representation of current flowing into theexemplary circuit of FIG. 4A; and

FIG. 5 illustrates a computer system that may implement various modulesof system.

DETAILED DESCRIPTION

The embodiments described below overcome the problems above by providinga system that is capable of performing more complete diagnostic analysisof a target transformer. Generally, the system is configured to driveone or more windings or phases of a target transformer with a sinusoidalAC voltage and to measure characteristic parameters of an excitationcurrent that flows into the windings/phases of the target transformer.The parameters are compared with parameters associated with so-calledbenchmark transformers or the previous data of the target transformer todetermine whether the target transformer matches the characteristics ofa given benchmark transformer. The benchmark transformers may includetransformers that are properly functioning transformers and transformersthat exhibit a particular kind of defect.

In operation, the parameters associated with the target transformer maybe compared with corresponding parameters associated with a benchmarktransformer that represents a properly functioning transformer or theprevious data of the target transformer to determine whether the targettransformer is in working order. On the other hand, the parametersassociated with the target transformer may match the parametersassociated with a benchmark transformer that, for example, has a shortedwinding or other defect. In this case, the target transformer may bedetermined to have a shorted winding.

FIG. 1 illustrates an exemplary system 100 for performing diagnostics ona transformer. The system 100 includes a voltage generator section 105,a switch section 110, a processor 120, and a storage device 125. Thevoltage generator section 105 includes a voltage source 106 and acurrent measuring device 107. The voltage source is configured togenerate a voltage that is typically lower than the voltage applied tothe target transformer when in normal in-service use. For example, thetest voltage may be about 12 kV. The resulting current into the targettransformer may be a non-sinusoidal AC current.

The current measuring device 107 is configured to measure the excitationcurrent flowing from the voltage source 106 to the winding of a targettransformer. The current measuring device 107 may include a currentsensing portion, such as a small resistance. Current flowing through theresistance results in a voltage drop across the resistance. The currentmeasuring device 107 may include analog-to-digital conversion circuitrythat samples the voltage developed across the resistance andcommunicates a digital representation of the sampled voltage to theprocessor 120.

The switch section 110 is configured to route the outputs of the voltagegenerator section 105 to different windings of a target transformer. Theswitch section 110 may correspond to a mechanical or solid-state switch.FIGS. 2A and 2B illustrate exemplary transformer configurations that maybe coupled to the switch section 110. FIG. 2A illustrates a Y-typetransformer 205 that includes an LTC/DETC 210. The switch section 110may couple a first voltage generator output to a center node, H0, of thetransformer 205. The switch section 110 may couple the other output toone of nodes H1, H2, and H3 to facilitate measuring the excitationcurrent flowing within one of windings H1-H0, H2-H0, or H3-H0. FIG. 2Billustrates a different transformer 215 that includes an LTC/DETC 220where access to a neutral node of the transformer may not be provided.In this case, the switch section 110 may couple the outputs of thevoltage generator section 105 to one of nodes H1, H2, and H3, tofacilitate measuring the excitation current flowing within one ofwindings/phases H1-H2, H1-H3, or H2-H3.

Returning to FIG. 1, the processor 120 is configured to compare thecurrent components associated with the exciting current with previouslydetermined component data associated with the target transformer orbenchmark transformers. The processor 120 may be in communication withthe voltage generator section 105 and the switch section 110 to controloperation of the respective sections. For example, the processor 120 maycontrol activation of the voltage generator 106 of the voltage generatorsection 105 and may control an output voltage and frequency of thevoltage generator section 105. The processor 120 may control the switchconfiguration of the switch section 110 to route voltage from thevoltage regulator section 105 to select a specific winding/phase of atarget transformer.

In some implementations, the processor 120 may be configured tocommunicate information and/or instructions to an operator. For example,the system 100 may include a display or a network interface thatfacilitates communication of instructions to an operator to have theoperator select a particular LTC/DETC position when testing theoperation of a target transformer. In alternative implementations, theprocessor 120 may be in control of a servo or other form of actuatorthat is coupled to the LTC/DETC of the target transformer to facilitateautomatic changing of the LTC/DETC position during testing.

Operations performed by the system 100 are illustrated in the flowdiagram of FIG. 3. To facilitate performance of the various operations,one or more non-transitory types of memories, such as RAM, ROM, flash,etc., may be in communication with the processor 120 and may storeinstruction code executable by the processor 120 to cause the processor120 to carry out all or part of the various operations.

At block 300, selection of an initial LTC/DETC position may beperformed. For example, an instruction for setting the LTC/DETC positionof the target transformer to an initial position, such as LTC/DETCposition 1, may be communicated to an operator via a display.

At block 305, the switch section 110 may be controlled to route voltageoutputs of the voltage generator section 105 to a first phase or windingof the target transformer. For example, the processor 120 may controlthe switch section 110 to select phase H1-H3 of the transformer, asillustrated in FIG. 2B.

At block 310, the voltage generator 106 of the voltage generator section105 may be energized at a first frequency, and the current phasor of theexciting current associated with the first frequency may be captured.For example, when energized the voltage generator 106 may generate a 12kV sinusoidal AC voltage at a frequency about 5% below a normaloperating frequency of the target transformer, such as 57 Hz when thenormal operating frequency is 60 Hz. A lower frequency may result in theinductance of the transformer becoming non-linear. Generation of thevoltage results in exciting current flow through the selectedwinding/phase of the target transformer.

The current measuring device 107 of the voltage generator section 105may measure the magnitude and phase of the current with respect to thephase of the voltage source to determine in-phase (I) and out-of-phase(Q) components of the current flowing into the target transformer. Forexample, the current measuring device 107 may digitally sample a voltagedeveloped across a sense resistor and based on the samples determine thein-phase (I) and out-of-phase (Q) components of the current. As notedabove, the current flowing into the target transformer may be anon-sinusoidal AC current. In this case, determination of the I and Qcomponents of the current may require determining the first harmonicwaveform of the measured current. For example, a filter or harmonicanalyzer may be utilized to determine the first harmonic.

FIG. 4A illustrates various currents flowing through-out an exemplarycircuit representation 400 of a single phase of a transformer. As shown,the transformer phase may be represented as the parallel combination ofinductor L_(m), resistor R_(m), and capacitor C. The voltage sourcesources current I_(m) into the parallel combination of components, whereI_(m) equals the sum of currents I_(L), I_(R), and I_(C), which flowrespectively through inductor L_(m), resistor R_(m), and capacitor C.

FIG. 4B illustrates a vector representation of the various currentcomponents. As shown, the in-phase current (I_(R)) corresponds to thecurrent I_(R), that flows through the equivalent resistance of thetransformer phase. The magnitude of the out-of-phase current (I_(Q))corresponds to the difference between the magnitudes of currents I_(L)and I_(C) that flow respectively through the equivalent inductor andcapacitor of the transformer phase.

Returning to FIG. 3, at block 315, the voltage generator 106 of thevoltage generator section 105 may be energized at a second frequency,and the in-phase (I) and out-of-phase (Q) components current phasor ofthe exciting current associated with the second frequency may becaptured. For example, the voltage generator 106 may be controlled togenerate a 12 kV sinusoidal AC voltage at a frequency about 5% above anormal operating frequency of the target transformer, such as 63 Hz whenthe normal operating frequency is 60 Hz. A higher frequency may resultin the inductance of the transformer becoming non-linear. The in-phase(I) and out-of-phase (Q) components of the current at this frequency maybe captured.

At block 320, the currents I_(L) and I_(C) flowing through inductorL_(m) and capacitor C, respectively, may be determined based on thein-phase (I) and out-of-phase (Q) component current measurementsperformed in blocks 310 and 315. In one implementation, the currentsassociated with the inductive and capacitive components are determinedaccording to the following methodology:

The quadrature component of the current measured at a lower frequency f₁(e.g., 60 Hz) may be expressed as:I _(Q1) =I _(L1) +I _(C1)  (1)

The quadrature component of the current measured at a higher frequencyf2 (e.g., 70 Hz) may be expressed as:I _(Q2) =I _(L2) +I _(C)  (2)

Using the applied voltage as reference, the inductive and capacitivecomponents can be expressed as follows:I _(L1) =V/jω ₁ L  (3)I _(C1) =jVω ₁ C  (4)I _(L2) =V/jω ₂ L  (5)I _(C2) =jVω ₂ C  (6)I _(L) =V/jωL  (7)I _(C) =jVωC  (8)where ω₁=2πf₁, ω₂=2πf₂, ω=2πf, f₁<f<f₂ and f is the power frequency.

Using,ω₁=ω−Δω  (9)ω₂=ω+Δω  (10)ξ=Δω/ω=Δf/f  (11)equations (3-8), (1) and (2) can be rewritten as:I _(Q1) =V/jω ₁ L+jVω ₁ C=I _(L)/(1−ξ)+I _(C)(1−ξ)  (12)

Similarly,I _(Q2) =I _(L)/(1+ξ)+I _(C)(1+ξ)  (13)

From (12):I _(L) =I _(Q1)(1−ξ)−I _(C)(1−ξ)²  (14)

Substituting (14) into (13) yields:I _(Q2) =I _(Q1)(1−ξ)/(1+ξ)+I _(C)[4ξ/(1+ξ)]  (15)

From (15),I _(C)=(1/4ξ)[I _(Q2)(1+ξ)−I _(Q1)(1−ξ)]  (16)

Substituting (16) into (14) yields:I _(L)=[(1−ξ²)/4ξ][I _(Q1)(1+ξ)−I _(Q2)(1−ξ)]  (17)

After calculation, the calculated currents, I_(L), I_(C), and measuredcurrent I_(R). may be stored to the storage device 125 and associatedwith the current LTC/DETC position and current phase as illustrated inTable 1.

TABLE 1 Phase Phase Phase Phase Phase Phase Phase Phase Phase LTC/DETC AB C A B C A B C Position I_(L) (mA) I_(L) (mA) I_(L) (mA) I_(C) (mA)I_(C) (mA) I_(C) (mA) I_(R) (mA) I_(R) (mA) I_(R) (mA) N 11.2 5.7 11.30.2 0.7 0.3 1.2 1.7 1.3 1L 23.3 18.0 23.3 1.3 1.0 1.3 22.3 1.0 2.3 2L11.3 5.8 11.3 0.3 0.8 1.3 1.3 0.8 1.3 . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .

Referring to the Table 1, a first record in the database may include thedata associated with a first combination of LTC/DETC position andwindings/phases. Similarly, a second record includes the data associatedwith a second combination of LTC/DETC position and windings/phase. Dataassociated with other bridging and non-bridging LTC/DETC positions maybe specified in additional records of the database.

The data may be represented differently to facilitate searching thedatabase according to the different types of patterns described above.For example, records in the database may be arranged to facilitatesearching the database for specific LTC/DETC patterns and/or phasepatterns. In this way, after a given pattern is determined for thetarget transformer, the database may be searched according to thepattern type. For example, a phase pattern may be determined for a givencombination of LTC/DETC position. The database may be searched for arecord associated with the same LTC/DETC position, and the values of therecord compared with the determined phase pattern.

When performing diagnostics on a target transformer, the data associatedwith the target transformer may be compared with data associated withone or more benchmark transformers. Comparison may be made on ameasurement-by-measurement basis or on a different basis. For example,current measurements and or the computed currents above associated withall three phases and for all LTC/DETC positions may be compared withcorresponding current measurements and or the computed currents of theone or more benchmark transformers. In this case, the currentmeasurements and/or computed currents for all combinations of phases andLTC/DETC positions would have to be performed on the target transformer.Once completed, the current measurements and/or computed currents wouldbe compared to determine whether the target transformer matches thecharacteristics of the benchmark transformer. In some instances,different weights may be applied to the various components to signifythe importance of one component over another. For example, I_(L)currents may be given a greater weight than I_(C) currents.

In some instances, iterations through the operations described in FIG. 3may terminate after an anomaly is first noticed with the targettransformer. For example, if an anomaly is detected when performingdiagnostics on a first LTC/DETC position, the operations may simplyterminate at that point instead of continuing through all other LTC/DETCpositions. For example, the calculations of the current components for agiven LTC/DETC may be performed and then compared with the currentcomponent data associated with a benchmark transformer when set to thesame LTC/DETC position. In this case, if the difference between themeasurements of the target transformer and the benchmark transformerexceeds a threshold, further analysis on the target transformer may bediscontinued.

Alternatively, the measurements may instead be compared to measurementsassociated with one or more different benchmark transformers to find abenchmark transformer that has similar current component characteristicsfor the selected LTC/DETC. This may, for example, be utilized todetermine a failure mode of the target transformer. For example, themeasurements associated with a given LTC/DETC position of the targettransformer may match a benchmark transformer that has a shorted windingon the same phase. If the measurements match between the targettransformer and the benchmark transformer, the target transformer may bedetermined to have a shorted winding.

Returning to FIG. 3, at block 340, if there are additional windings orphases to measure, the next winding or phase is selected and theoperations repeat from block 310. For example, a three-phase transformerhas three windings or sets of windings for each phase. In this case, thecurrent components associated with exciting current flow through eachphase or set of windings would be measured.

If at block 340, the measurements have been completed on all the phases,then at block 350, if there are additional LTC/DETC positions tomeasure, the next LTC/DETC position may be selected and the operationsmay repeat from block 305. For example, if the target transformer has anLTC/DETC with 16 positions, each position may be selected and themeasurements described above performed for all 16 positions. TheLTC/DETC positions may include both non-bridging positions and bridgingpositions, which are positions where two adjacent taps of the targettransformer are connected via a preventative autotransformer.

If at block 350 all the LTC/DETC positions have been evaluated, thendiagnosis of the target transformer may be completed, as represented byblock 370. As noted above in block 320, comparison of the measurementsassociated with the target transformer to measurements associated with abenchmark transformer may occur after all combinations of LTC/DETCpositions and windings/phases have been measured. In this case,comparison may occur at block 370. Alternatively, comparison may beperformed after each LTC/DETC position and phase has been exercised.

FIG. 5 illustrates a computer system 500 that may correspond to theprocessor 120 or form part of any of the modules referenced herein. Thecomputer system 500 may include a set of instructions 545 that theprocessor 505 may execute to cause the computer system 500 to performany of the operations described above. The computer system 500 mayoperate as a stand-alone device or may be connected, e.g., using anetwork, to other computer systems or peripheral devices.

In a networked deployment, the computer system 500 may operate in thecapacity of a server or as a client-user computer in a server-clientuser network environment, or as a peer computer system in a peer-to-peer(or distributed) network environment. The computer system 500 may alsobe implemented as or incorporated into various devices, such as apersonal computer or a mobile device, capable of executing theinstructions 545 (sequential or otherwise) that specify actions to betaken by that machine. Further, each of the systems described mayinclude any collection of sub-systems that individually or jointlyexecute a set, or multiple sets, of instructions to perform one or morecomputer functions.

The computer system 500 may include one or more memory devices 510 on abus for communicating information. In addition, code operable to causethe computer system to perform any of the operations described above maybe stored in the memory 510. The memory 510 may be a random-accessmemory, read-only memory, programmable memory, hard disk drive or anyother type of memory or storage device.

The computer system 500 may include a display 530, such as a liquidcrystal display (LCD), a cathode ray tube (CRT), or any other displaysuitable for conveying information. The display 530 may act as aninterface for the user to see the functioning of the processor 505, orspecifically as an interface with the software stored in the memory 510or in the drive unit 515.

Additionally, the computer system 500 may include an input device 525,such as a keyboard or mouse, configured to allow a user to interact withany of the components of system 500.

The computer system 500 may also include a disk or optical drive unit515. The disk drive unit 515 may include a computer-readable medium 540in which the instructions 545 may be stored. The instructions 545 mayreside completely, or at least partially, within the memory 510 and/orwithin the processor 505 during execution by the computer system 500.The memory 510 and the processor 505 also may include computer-readablemedia as discussed above.

The computer system 500 may include a communication interface 535 tosupport communications via a network 550. The network 550 may includewired networks, wireless networks, or combinations thereof. Thecommunication interface 535 network may enable communications via anynumber of communication standards, such as 802.11, 802.12, 802.20,WiMax, cellular telephone standards, or other communication standards.

Accordingly, the method and system may be realized in hardware,software, or a combination of hardware and software. The method andsystem may be realized in a centralized fashion in at least one computersystem or in a distributed fashion where different elements are spreadacross several interconnected computer systems. Any kind of computersystem or other apparatus adapted for carrying out the methods describedherein may be employed.

The method and system may also be embedded in a computer programproduct, which includes all the features enabling the implementation ofthe operations described herein and which, when loaded in a computersystem, is able to carry out these operations. Computer program in thepresent context means any expression, in any language, code or notation,of a set of instructions intended to cause a system having aninformation processing capability to perform a particular function,either directly or after either or both of the following: a) conversionto another language, code or notation; b) reproduction in a differentmaterial form.

While methods and systems have been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope. In addition, many modifications may be made toadapt a particular situation or material to the teachings withoutdeparting from its scope. Therefore, it is intended that the presentmethods and systems not be limited to the particular embodimentdisclosed, but that the disclosed methods and systems include allembodiments falling within the scope of the appended claims.

I claim:
 1. A method comprising: controlling, by a processor ofdiagnostic equipment, a voltage source to output a voltage to a targettransformer at a first frequency; measuring a magnitude and a phaseangle of a first excitation current of the target transformer withrespect to the voltage at the first frequency; controlling, by theprocessor of diagnostic equipment, the voltage source to output thevoltage to the target transformer at a second frequency, the secondfrequency being greater than or less than the first frequency; measuringa magnitude and a phase angle of a second excitation current of thetarget transformer with respect to the voltage at the second frequency;determining, as a function of the magnitude and phase angle of the firstexcitation current and the second excitation current, an inductivecomponent, a capacitive component, or both for the target transformer;determining, by the processor of diagnostic equipment, a pattern for thetarget transformer, the pattern formed by the inductive component, thecapacitive component, or both; comparing, by the processor of diagnosticequipment, the pattern formed by the inductive component, the capacitivecomponent, or both with another pattern previously determined for thetarget transformer, a benchmark transformer, or both; and outputting, bythe processor of diagnostic equipment, a diagnostic analysis of thetarget transformer.
 2. The method of claim 1, wherein the firstfrequency and the second frequency are different than a nominaloperating frequency of the target transformer.
 3. The method of claim 1,wherein the voltage at the first frequency and the voltage at the secondfrequency are equal.
 4. The method of claim 1, wherein determining, bythe processor of diagnostic equipment, the pattern further comprisesapplying different weights to the inductive component, the capacitivecomponent, or both.
 5. The method of claim 1, wherein comparing, by theprocessor of diagnostic equipment, the pattern comprises identifying, bythe processor of diagnostic equipment, the benchmark transformer, thebenchmark transformer identified by matching the determined pattern forthe target transformer to a pattern associated with the benchmarktransformer.
 6. The method of claim 5, wherein the benchmark transformerhas predetermined electrical characteristics indicative of a diagnosisof an operational characteristic of the target transformer.
 7. Themethod of claim 5, wherein the pattern associated with the benchmarktransformer is formed by a plurality of magnitudes of an inductivecomponent, a capacitive component, or both, of the benchmarktransformer.
 8. The method of claim 1, wherein the target transformer isa three-phase transformer.
 9. The method of claim 1, wherein the targettransformer includes a load-tap-changer (LTC) or a de-energized tapchanger (DETC), wherein the pattern of the target transformer isdetermined for each position of the LTC or DETC.
 10. A system forperforming diagnostics on a target transformer, the system comprising:an input configured to receive voltage from a voltage source; a switchcircuitry configured to selectively apply the voltage to each of aplurality of phases of the target transformer; a current sensorconfigured to sense a current flowing in each of the phases of thetarget transformer; a processor in communication with the voltagesource, the switch circuitry, and the current sensor; and non-transitorycomputer readable media in communication with the processor that storesinstruction code executable by the processor, the instruction codecomprising: instructions executable by the processor to control theswitch circuitry to apply the voltage to each phase of the targettransformer; for each phase: instructions executable by the processor tocontrol the voltage source to output voltage at a first frequency and asecond frequency; instructions executable by the processor to measure,via the current sensor, a magnitude and a phase angle of a firstexcitation current of the target transformer at the first frequency, anda magnitude and a phase angle of a second excitation current of thetarget transformer at the second frequency; instructions executable bythe processor to determine inductive components, capacitive components,or both, for each of the plurality of phases of the target transformerbased on the first and second excitation currents; instructionsexecutable by the processor to determine a pattern of the inductivecomponents, capacitive components, or both; instructions executable bythe processor to compare the pattern to another pattern previouslydetermined for the target transformer, a benchmark transformer, or both;and instructions executable by the processor to output a diagnosticanalysis of the target transformer.
 11. The system of claim 10, whereinthe target transformer includes a load-tap-changer (LTC) or de-energizedtap changer (DETC), wherein the instructions executable by the processorcomprise instructions executable by the processor to change theload-tap-changer (LTC) or de-energized tap changer (DETC) from one tapto another, and the instructions to determine a pattern of the inductivecomponents, capacitive components, or both further comprise instructionsto determine the pattern for the phases for each of a plurality of taps.12. The system of claim 11, wherein the another pattern is a storedrecord comprising inductive components, capacitive components, or both,for the plurality of phases of the target transformer.
 13. The system ofclaim 11, wherein the another pattern is a stored record comprisinginductive components, capacitive components, or both, for a plurality ofphases of the benchmark transformer.
 14. The system of claim 10, whereinthe voltage at the first frequency and the voltage at the secondfrequency are equal.
 15. The system of claim 10, wherein the voltage atthe first frequency is above a nominal operating frequency of the targettransformer and the second frequency is below the nominal operatingfrequency of the target transformer.
 16. The system of claim 10, whereinthe instructions executable by the processor to determine a pattern ofthe inductive components, capacitive components, or both comprisesinstructions executable by the processor to determine a pattern of theinductive components, capacitive components, or both for the pluralityof phases.
 17. A non-transitory machine-readable storage medium havingstored thereon a computer program comprising instructions for performingdiagnostics on a target transformer, the machine readable storage mediumcomprising instructions to control a voltage source to output a voltageto the target transformer at a first frequency; instructions to receivea measurement of a magnitude and a phase angle of a first excitationcurrent of the target transformer with respect to the voltage at thefirst frequency; instructions to control the voltage source to outputthe voltage to the target transformer at a second frequency, the secondfrequency being greater than or less than the first frequency;instructions to receive a measurement of a magnitude and a phase angleof a second excitation current of the target transformer with respect tothe voltage at the second frequency; instructions to determine, as afunction of the magnitude and phase angle of the first excitationcurrent and the second excitation current, an inductive component, acapacitive component, or both for the target transformer; instructionsto determine a pattern for the target transformer, the pattern formed bythe inductive component, the capacitive component, or both; instructionsto compare the pattern formed by the inductive component, the capacitivecomponent, or both with another pattern previously determined for thetarget transformer, a benchmark transformer, or both; and instructionsto output a diagnostic analysis of the target transformer.
 18. Themachine readable storage medium of claim 17, wherein the instructions todetermine the pattern further comprises instructions to apply differentweights to the inductive component, the capacitive component, or both ofthe first excitation current, the second excitation current, or both.19. The machine readable storage medium of claim 17, whereininstructions to compare the pattern comprises instructions to search adatabase to identify the benchmark transformer by comparison of thedetermined pattern for the target transformer to the another patternassociated with the benchmark transformer.
 20. The machine readablestorage medium of claim 17, wherein instructions to compare the patterncomprises instructions to search a database to identify the anotherpattern as a previously stored pattern of the target transformer.